As demands for secondary batteries have increased with the IT mobile market growth, applications of secondary batteries have gradually expanded to electric vehicles and energy storage systems. Particularly, development of next generation lithium batteries having energy density higher than lithium ion batteries (maximum energy density ˜250 Wh/kg) is required in order to obtain batteries having high energy density such as batteries for electric vehicles, and one of secondary batteries according with such requirements is a lithium metal battery.
A lithium metal battery is a secondary battery using lithium metal as a negative electrode, and has been researched and developed in various forms such as a lithium-air battery or a lithium-sulfur battery.
Lithium has very low standard reduction potential of −3.045 V SHE (Standard Hydrogen Electrode), has rather high specific gravity of 1.85 cm3/g, and has energy density per weight (3860 mAh/g) of 10 or more times higher than currently commercialized carbon-based negative electrodes (372 mAh/g), and is an ideal material enabling a battery to have high energy density.
However, using lithium metal as a negative electrode of a secondary battery causes problems as follows. First, lithium metal has high reactivity with a liquid electrolyte component forming a passivation layer on the lithium metal surface when bringing the liquid electrolyte into contact with the lithium metal due to spontaneous decomposition of the electrolyte. Such a layer causes deintercalation and collapse of the passivation layer with the progress of continuous charge and discharge cycles of the lithium metal battery, and, as the passivation layer is additionally produced in the gap produced from the phenomenon, causes a problem of degenerating a battery life time property by forming so-called ‘dead lithium (dead Li)’. In addition, the passivation layer causes a local current density difference forming dendritic lithium dendrite while non-uniformizing current distribution during charge. In addition, when dendrite formed as above continuously grows breaking through a separator and is brought into contact with a positive electrode, an internal short circuit occurs causing a phenomenon of battery explosion.
Second, lithium is an alkali metal and has high reactivity with moisture, and, even when moisture is included in an electrolyte in a few ppm level, may generate heat and gas by reacting with the moisture causing a problem in battery stability due to internal expansion of a battery. Third, lithium is highly soft and has weak mechanical strength, and handling properties are very poor to use without additional surface treatment. Accordingly, technologies to stabilize a lithium metal electrode and to suppress dendrite formation are core technologies prerequisite to the development of next generation lithium secondary batteries.
In order to resolve such problems, studies such as introducing a polymer protective layer or an inorganic solid protective layer to a lithium metal layer, increasing a concentration of a salt of a liquid electrolyte, or using proper additives have been progressed currently. However, effects of lithium dendrite suppression of such studies are insignificant. Accordingly, modifying a form of a lithium metal negative electrode itself or modifying a structure of a battery may become an effective alternative in resolving the problems.